3.2 Forces in Action

when a resultant force acts on a body with mass

the gravitational force acting upon the object, F=mg

the force that arises when two surfaces rub against each other

the resistive force on an object travelling through a fluid (e.g. water or air)

the force within a stretched cable or rope

the upward buoyancy force acting on an object when it is in a fluid

the force arising when an object rests against another object acting at a 90° angle to the plane of contact

a diagram that models all forces acting on an object - each force is represented as a vector arrow, scaled to the magnitude of the force they represent, and pointing in the direction that the force acts. Each arrow should be labelled with the name of the force it represents

resolving forces with their horizontal and vertical components

the weight acts vertically down, friction acts parallel to the slope, normal reaction force acts perpendicular to slope - resolve forces parallel to slope rather than horizontally and vertically

the speed of the object, the object’s shape and texture, and the density of the fluid

the speed of the object, and the object’s cross-sectional area

the square of the speed, it also increases as cross sectional area increases

acceleration will be g but drag forces will reduce the net force, reducing acceleration therefore as drag is proportional to the square of the speed, the drag force will increase until it is equal in magnitude to the weight of the object

during free fall, as the weight of the object stays constant and resultant force decreases until 0, at a point where drag force = weight, the object will reach a maximum velocity (terminal velocity)

1. wrap elastic bands around the tube of viscous liquid at set intervals measured by the ruler

2. drop the ball into the tube and record the time taken between bands with a stopwatch

3. repeat 4 times to reduce the effect of random errors

4. use a magnet to remove metal ball

use time taken between rubber bands to work out velocity

plot a graph of velocity against time

the velocity that the graph tends to is terminal velocity

magnitude of the force, multiplied by the perpendicular distance/turning effect of force about a point

a pair of forces, that have equal magnitude and opposite direction, applied to a body in parallel with each other, they produce a rotational force about the central pivot point

total moment of the couple, magnitude of one of the forces x perpendicular distance between forces

for a body in equilibrium, the sum of the clockwise moments about any point is equal to the sum of the anticlockwise moments about the same point

the point where the entire weight of the object appears to act

a freely suspended object will come to rest with its centre of mass vertically below its suspension point, so a plumb line can be used to determine the centre of mass of an object, take multiple lines, the intersection is the centre of mass

the mass per unit volume

use a scale

for liquid, a cylinder, for a regular solid, vernier callipers, for an irregular solid, a eureka can (volume of water displaced is equal to volume of solid)

the normal force exerted on a surface per unit cross sectional area

the upthrust exerted on a body immersed in fluid, whether partially or fully submerged, is equal to the weight of the fluid that the body displaces

if the upthrust acting up is smaller than the weight acting down

the weight is equal to the upthrust, so the object’s weight is equal to the weight of the fluid it displaces